N.sup.5 -Methyl-5,6,7,8-tetrahydrofolic acid, herein sometimes abbreviated with the denotation N.sup.5 -methyl-THF, is the predominant circulating form of reduced folates in mammals.
There exists an increasing interest for the application of this natural metabolite as at least one active compound in a therapeutical agent, for example as vitamin in folate deficiency states. Therapeutical agents, containing N.sup.5 -methyl-THF, may also be used for the synergistic exertion of influence of a cancer controlling compound and/or for the reduction of the toxicity of a cancer controlling compound and/or for the protection of human and/or animal cells. N.sup.5 -Methyl-THF in such an agent is usually contained in an amount from 1 mg to 500 mg, especially from 5 mg to 150 mg, per dosis unit. The medicament is preferably in the form of a parenteral and/or oral pharmaceutical preparation.
In this regard, reference is made to Cortellaro M., Boschetti C. and Polli E., "High Dose Methotrexate with N.sup.5 -methyl-THF Rescue in Acute Leukemia and Hodgkin's Lymphoma" Chemioterapia Oncologica 2 (N.4)(Dec. Suppl./1978)311; Mazzei T. et al., "High Dose Methotrexate Therapy in High-Risk Trophoblastic Tumors" ibid. 289; Novelli A. et al., "Clinical Data on Rescue of High Dose Methotrexate With N.sup.5 -methyl-THF in Human Solid Tumors" ibid 289; Reggev A. and Djerassi I., "Rescue from High-Dose Methotrexate With N.sup.5 -methyl-THF" Cancer Treat. Rep. (1986) 70,251; and Nigra E. et al., "Can Folates Improve the Efficacy of 5-FU in a Polychemotherapy Schedule? A New Treatment for Advanced Colorectal Cancer." Fifth NCI-EORT Symposium on New Drugs in Cancer Therapy, Oct. 22-24, 1986, Amsterdam (Oncology Abstracts (1987) 2, 273.
The absolute configuration of the natural form of N.sup.5 -methyl-THF corresponds to the following formula A ##STR1## and is defined as (6S, L), where "6S" refers to the configuration at C.sub.6 of the pterin-ringsystem and "L" refers to the configuration of the .alpha.-carbon atom of the glutamate side chain.
The commercially available forms of N.sup.5 -methyl-THF, obtained from the chemical reduction of folic acid, are a mixture of the (6RS)-diastereoisomers with the "L"-configuration in the glutamate side chain.
There exists no agreement about the function of the (6R)-diastereoisomer of N.sup.5 -methyl-THF. It has been assumed, that the unnatural (6R)-diastereoisomer of N.sup.5 -methyl-THF is inert and is excreted as such in an unchanged form; see, for example, D. G. Weir et al., "The Absorption of the Disastereoisomers of N.sup.5 -methyl-THF in Man: A Carrier-Mediated Process." Clinical Science and Molecular Medicine (1973) 45, 625. But there has also been postulated, that the unnatural (6R)-diastereoisomer could interfere to the folate transport system through the cell membranes of mammals, including humans; see, for example, Chello P. L. et al., "Further Studies of Stereospecificity at Carbon 6 for Membrane Transport of Tetrahydrofolates". Biochem. Pharmacol. (1982) 31, 1527; White J. C. et al., "Lack of Stereospecificity at Carbon 6 of N.sup.5 -methyl-THF Transport in Ehrlich Ascite Tumor Cells." Journal of Biol. Chem. (1978) 253, 242-245; and White J. C. and I. D. Goldman, "Lack of Stereospecificity at Carbon 6 of N.sup. 5 -methyl-THF Transport: Possible Relevance to Rescue Regimens with Methotrexate and Leucovorin." Chemistry and Biology of Pteridines (Kisliuk/Brown eds./1979), 625.
The individual (6S) and (6R)-diastereoisomers of N.sup.5 -methyl-THF have been prepared in milligramm amounts of doubtful chemical purity, either by complex enzymatic reactions (see, for example, Chello P. L. et al., "Further Studies of Stereospecificity at Carbon 6 for Membrane Transport of Tetrahydrofolates," Biochem. Pharmacol, (1982) 31, 1527), or indirect by the reduction of the corresponding N.sup.5, N.sup.10 -methylene-THF isomer, which has been obtained by chromatographical separation of the (6RS)-mixture (see White J. C. et al., "Lack of Stereospecificity at Carbon 6 of N.sup.5 -methyl-THF Transport in Ehrlich Ascite Tumor Cells." Journal of Biol. Chem, (1978) 253, 242-245; and Kaufman B. T. et al., "Chromatographic Separation of the Disastereoisomers of dl,L-5,10-methylenetetrahydrofolate." Journal of Biol. Chem. (1963) 238, 1498.
5, 6, 7, 8-Tetrahydrofolic acid is herein sometimes abbreviated with the denotation THF.